1. Field of the Invention
The present invention relates to a helical antenna used as an antenna for a mobile terminal in a mobile radio communication system or the like using a mobile satellite and to a method for producing the helical antenna.
2. Description of the Related Art
A mobile radio communication system using the mobile satellite in general uses a frequency band of 1.985 to 2.015 GHz as a transmission frequency band and a frequency band of 2.17 to 2.2 GHz as a reception frequency band.
In transmission and reception between the mobile satellite and a mobile station, therefore, an antenna having a frequency characteristic capable of effectively performing transmission and reception with a low return loss in a frequency band of about 30 MHz is required.
And a small-sized and lightweight antenna is necessary as an antenna for a mobile terminal.
Thus a helical antenna is used, but in case that such an antenna is made small-sized in axial length and in diameter, its transmission frequency band results in being narrow.
For example, a 4-wire wound helical antenna of about 1/4 to 5/4 wavelengths in axial length and of about 0.1 wavelength in diameter can cover only such a very narrow frequency band as 1 to 2% of a frequency band to be used.
Due to this, such an antenna as this is unsuitable for an antenna using two different frequency bands, for example, a frequency band of 1.985 to 2.015 GHz and a frequency band of 2.17 to 2.2 GHz like an antenna used in a mobile radio communication system using a mobile satellite.
FIG. 14 is a characteristic diagram showing a relation between frequency and return loss in case that a helical antenna adjusted to a frequency band of 1.985 to 2.015 GHz is used in both frequency bands of 1.985 to 2.015 GHz and 2.17 to 2.2 GHz.
In FIG. 14, a .DELTA.-mark 96 indicates a return loss at a frequency of 1.985 GHz and a .DELTA.-mark 97 indicates a return loss at a frequency of 2.015 GHz.
And a .DELTA.-mark 98 indicates a return loss at a frequency of 2.17 GHz and a .DELTA.-mark 99 indicates a return loss at a frequency of 2.2 GHz.
As clearly known from FIG. 14, this antenna can cover transmission and reception in a frequency band of 1.985 to 2.015 GHz, but cannot cover transmission and reception in a frequency band of 2.17 to 2.2 GHz.
FIG. 15 is a structural diagram showing a conventional helical antenna capable of covering the above-mentioned two frequency bands and a feeder circuit of it.
In FIG. 15, an 8-wire wound antenna body 90 forming the helical antenna is flatly unrolled to be shown.
An 8-wire wound helical antenna capable of covering two frequency bands is formed by winding this antenna body 90 around the outer circumferential surface of a cylindrical body, not illustrated, made of a dielectric material of polycarbonate or the like.
The antenna body 90 is composed of a film 902 formed in the shape of a parallelogram out of a dielectric sheet made of polyimide or the like, first antenna elements 904 composed of conductive wires which extend on one surface of this film 902 in the long-side direction of said film 902 at a specified pitch angle and are arranged in parallel with one another at specified intervals in the short-side direction of said film 902, and second antenna elements 906 shorter than the first antenna elements 904.
The first antenna elements 904 and the second antenna elements 906 are arranged alternately with each other in the short-side direction of the film 902 in a state where their lower ends are arranged in a line.
In this case the first antenna elements 904 are adjusted in length to a frequency band of 1.985 to 2.015 GHz and the second antenna elements 906 are adjusted in length to a frequency band of 2.17 to 2.2 GHz.
The feeder circuit 92 is composed of a feeder system 94 of a first frequency band F1 (of 1.985 to 2.015 GHz) and a feeder system 96 of a second frequency band F2 (of 2.17 to 2.2 GHz).
The feeder system 94 of the first frequency band F1 is composed of a dividing/synthesizing circuit 941 which divides a high-frequency signal into two high-frequency signals being different by 180 degrees in phase from each other or synthesizes two high-frequency signals being different by 180 degrees in phase from each other into a high-frequency signal, a dividing/synthesizing circuit 942 which divides one high-frequency signal obtained by division performed by this dividing/synthesizing circuit 941 into two high-frequency signals (of 0 degree and -90 degrees) being different by 90 degrees in phase from each other to feed them to the antenna body 90 or synthesizes two high-frequency signals (of 0 degree and -90 degrees) being different by 90 degrees in phase from each other given from the antenna body 90 into a high-frequency signal, and a dividing/synthesizing circuit 943 which divides the other high-frequency power obtained by division performed by the dividing/synthesizing circuit 941 into two high-frequency signals (of -180 degrees and -270 degrees) being different by 90 degrees in phase from each other to feed them to the antenna body 90 or synthesizes two high-frequency signals (of -180 degrees and -270 degrees) being different by 90 degrees in phase from each other given from the antenna body 90 into a high-frequency signal.
Each of the input/output terminals of the dividing/synthesizing circuits 942 and 943 is connected with each of the first antenna elements 904 of the antenna body 90 through a coupling wire 944.
Number 945 indicates a connecting terminal to a transmission/reception system of the feeder system 94 of the first frequency band F1.
The feeder system 96 of the second frequency band F2 is composed of a dividing/synthesizing circuit 961 which divides a high-frequency signal into two high-frequency signals being different by 180 degrees in phase from each other or synthesizes two high-frequency signals being different by 180 degrees in phase from each other into a high-frequency signal, a dividing/synthesizing circuit 962 which divides one high-frequency signal obtained by division performed by this dividing/synthesizing circuit 961 into two high-frequency signals (of 0 degree and -90 degrees) being different by 90 degrees in phase from each other to feed them to the antenna body 90 or synthesizes two high-frequency signals (of 0 degree and -90 degrees) being different by 90 degrees in phase from each other given from the antenna body 90 into a high-frequency signal, and a dividing/synthesizing circuit 963 which divides the other high-frequency signal obtained by division performed by the dividing/synthesizing circuit 961 into two high-frequency signals (of -180 degrees and -270 degrees) being different by 90 degrees in phase from each other to feed them to the antenna body 90 or synthesizes two high-frequency signals (of -180 degrees and -270 degrees) being different by 90 degrees in phase from each other given from the antenna body 90 into a high-frequency signal.
Each of the input/output terminals of the dividing/synthesizing circuits 962 and 963 is connected with each of the second antenna elements 906 of the antenna body 90 through a coupling wire 964.
Number 965 indicates a connecting terminal to a transmission/reception system of the feeder system 96 of the second frequency band F2.
In a conventional helical antenna composed as described above, at the time of transmission, when a high-frequency signal of the first frequency band F1 is supplied from the transmission system to the terminal 945 of the feeder system 94, this high-frequency signal is divided by the dividing/synthesizing circuits 941, 942 and 943 into four high-frequency signals respectively having phase differences of 0, -90, -180 and -270 degrees to be fed to the respective first antenna elements 904 of the antenna body 90, and is radiated as radio-waves.
And when a high-frequency signal of the second frequency band F2 is supplied from the transmission system to the terminal 965 of the feeder system 96, this high-frequency signal is divided by the dividing/synthesizing circuits 961, 962 and 963 into four high-frequency signals respectively having phase differences of 0, -90, -180 and 270 degrees to be fed to the respective second antenna elements 905 of the antenna body 90, and is radiated as radio-waves.
On the other hand, among radio-waves receiving at the helical antenna, the radio-waves in the first frequency band F1 are caught by the first antenna elements 904 of the antenna body 90, and high-frequency powers generated in the first antenna elements 904 are synthesized in sequence by the dividing/synthesizing circuits 943, 942 and 941 and are supplied to the reception system through the terminal 945.
And among radio-waves receiving at the helical antenna, the radio-waves in the second frequency band F2 are caught by the second antenna elements 906 of the antenna body 90, and high-frequency powers generated in the second antenna elements 906 are synthesized in sequence by the dividing/synthesizing circuits 963, 962 and 961 and are supplied to the reception system through the terminal 965.
However, a conventional helical antenna has a structure where two sets of antenna elements, one of which sets comprises four conductive wires adjusted in length correspondingly to one of the two frequency bands and the other of which sets comprises four conductive wires adjusted in length correspondingly to the other of the two frequency band, are combined and these sets of antenna elements are provided with the respective feeder systems. As clearly known from FIG. 13 also, in order to cover the two frequency bands, six dividing/synthesizing circuits are needed in addition to two feeder connectors corresponding to the number of feeder systems and eight connecting points for the respective conductive wires of the helical antenna.
Therefore, since such feeder circuits can be mounted only two-dimensionally on a printed circuit board, the conventional helical antenna has some problems that the printed circuit board and the feeder circuit portion become large-sized, complicated and expensive.
And it is very difficult also to arrange eight connecting pins or the like for connecting respectively the conductive wires of the helical antenna and the dividing/synthesizing circuits with each other closely to the supporting board of the helical antenna.